1. Field of the Invention
The present invention relates to a method for antenna selection such as is applied in the area of radio/high frequency identification systems.
2. Description of the Related Art
In many technical applications, it is useful to control the active phase of an antenna. Particularly in the area of radio/high frequency identification systems, i.e. systems for identifying objects and living organisms by RFID, or transponder, technologies, it is often necessary, for example, to achieve a selection result by multiple transmitting and/or receiving. The number of such RFID systems has significantly increased over the last few years; they serve for a contactless identification of objects or products, for example.
The transmitting frequencies of those systems are mainly in the ISM frequency bands which may be used by industry, science and medical technology without a license. In this context, the special feature of transponder technology is that the transponders and/or receivers, which are usually passive, draw the energy which is necessary for their function from the field used for transmitting and receiving data. Depending on the function principle, one distinguishes between electromagnetic and inductive systems, both systems using different kinds of antennas for the efficient coupling of a transmitter and a receiver. In inductive coupling, for example, in a simple example, an antenna will comprise a conductor loop, wherein the voltage induced in the conductor loop by a magnetic field present throughout the loop may be evaluated as a signal. Since the magnitude of the induced voltage depends on the magnetic flux present throughout the entire conductor loop, it is necessary, for a maximum coupling of the transponder to the magnetic field, that the area normal of the area formed by the conductor loop be parallel to the field direction of the magnetic field. If the transmitter generates the magnetic field also by a coil, the area normals of the transmitting coil and the receiving coil of the transponder are ideally parallel with each other to achieve the best possible coupling between the transmitter and the receiver. In the worst case in which the area normal of the transmitting coil is orthogonal to the area normal of the receiving coil, no direct data transmission is possible anymore. In most cases, RFID systems work with only one antenna, which is the reason why, due to the fact described above, a selection is possible only in one space direction. There are application areas, however, in which, due to the orientation of one or more transponders, a single antenna is not sufficient. In this context, one consider, for example, a pallet full of products to be sensed, which are arranged in different orientations on the pallet. In such cases, in inductive systems, for example, the shape of the antenna may be altered by employing special spatially-shaped wire windings. Furthermore, several antennas which are spatially separated and arranged in different orientations with respect to one another may be employed, wherein switching between those is performed according to a predetermined established switching pattern.
By established switching patterns in switching between several antennas, only antenna frame times which are the time intervals in which an antenna is active having a fixed period or fixed time frame, may be adjusted, wherein the antenna frame time must be larger than the duration of the longest transmitting and/or receiving operation to be expected. Thereby, long read/write times develop during repeating read/write operations by means of different antennas. If the illumination of four space directions by four antennas is necessary, for example, a single read/write operation will need four times the transmission time of the read/write operation of maximum duration, irrespectively of how long the transmission time of the data to be transmitted actually is.